“We once again openly declare that no woman was involved at Westgate,” Shebab said on Twitter, reiterating it had a policy of “not employing sisters for such missions”.
“A week after Westgate, the Kenyan government and Western intelligence officials failed to uncover the facts and details of the Westgate Operation,” it added, playing up an apparent lack of information on the attackers and details of how the massacre was planned and carried out.
“The Kenyan govt (government) is still chasing its tail by holding on to the hopeless notion that a woman led the attack,” the group said in another Twitter post.
The four-day bloodbath at the upmarket shopping mall, which Kenyan forces brought to an end on Tuesday, left at least 67 people dead. The Kenyan Red Cross said Monday that 39 more were still missing.
Lewthwaite, a 29-year-old Muslim convert, was married to Germaine Lindsay, one of four suicide bombers who attacked the London transport network in July 2005, killing 52 people.
The mother-of-three has been on the run in East Africa for around two years and is wanted by Kenyan police for alleged involvement in a separate terror plot.
Interpol issued a “red notice” arrest warrant for Lewthwaite on charges of possessing explosives and conspiracy to commit a felony dating back to December 2011.
Kenyan officials have given contradictory statements about whether a British woman may have been involved in the attack